Carrier recovery in a QAM receiver

Abstract

The present invention relates to inspection apparatus for use in wellbores in the oil and gas industries. In particular the invention relates in general to the field of transmission of data between downhole module in a wellbore and a controlling module at the surface. The invention provides a method an apparatus for receiving data from a telemetry module, the data being modulated according to a quadrature amplitude modulation coding scheme, in which a receiver has a carrier recovery circuit comprising a phase detector, a loop filter and a signal synthesizer generating an inphase and a quadrature demodulation signal. The method comprises receiving a preamble sinusoidal signal representing a symbol; determining the cross product of the received inphase and quadrature components and ideal inphase and quadrature components; indexing a look up table with said cross product to determine a phase adjustment and adjusting the phase of the demodulation signals in accordance with said determined phase adjustment.

The invention claimed is: 
     
       1. A method of receiving data from a telemetry module, the data being modulated according to a quadrature amplitude modulation coding scheme, in which a receiver has a carrier recovery circuit comprising a phase detector, a loop filter and a signal synthesiser generating an inphase and a quadrature demodulation signal, the method comprising the steps of:
 a) receiving a preamble sinusoidal signal representing a symbol; 
 b) matching a phase of the inphase signal generated by the signal synthesiser to a phase of the received signal; 
 c) matching a phase of the quadrature signal generated by the signal synthesiser to be ninety degrees out of phase the phase with the received signal; 
 d) repeating a step of determining whether the received signal represents a symbol having positive inphase and quadrature components; in the event the inphase and quadrature components of the received signal are not both positive, adjusting the phase of the inphase and quadrature components by a fixed phase shift until the received signal represents a symbol having positive inphase and quadrature components; 
 e) determining a cross product of the received inphase and quadrature components and ideal inphase and quadrature components; 
 f) indexing a look up table with said cross product to determine a phase adjustment; 
 g) adjusting the phase of the demodulation signals in accordance with said determined phase adjustment and further comprising adjusting for small phase rotations due to frequency fluctuations by 
 h) determining whether the received signal represents a symbol having equal inphase and quadrature components; 
 i) in the event the received signal represents a symbol having equal inphase and quadrature components, updating a phase difference measure in dependence upon the difference between the magnitude of the inphase and quadrature component of the received signal; and 
 j) adjusting the phase of the signals generated by the signal synthesiser when said phase difference measure exceeds a predetermined threshold. 
 
     
     
       2. The method according to  claim 1 , further comprising repeating steps e) to g) until the phase adjustment is less than a predetermined minimum. 
     
     
       3. The method according to  claim 2 , in which said predetermined minimum is 0.5 degrees. 
     
     
       4. The method according to  claim 1 , in which the preamble signal is a sinusoidal signal representing a symbol in the encoding scheme having the greatest signal to noise ratio. 
     
     
       5. The method according to  claim 1 , in which the lookup table comprises entries for unequally spaced phase differences with entries close together for small phase differences and entries further apart for greater phase differences. 
     
     
       6. The method according to  claim 1 , in which the phase difference measure is updated at step i) by incrementing the phase difference measure by one if a first one of the components is greater than a second one of the components and by decrementing the phase difference measure by one if the second one of the components is greater that the first one of the components. 
     
     
       7. The method according to  claim 6  in which the predetermined threshold is fifteen. 
     
     
       8. The method according to  claim 1 , in which the phase is adjusted by step j) by 0.5 degrees when said phase difference measure exceeds a predetermined threshold. 
     
     
       9. An apparatus for receiving data from a telemetry module comprising a receiver having a carrier recovery circuit comprising a phase detector, a loop filter and a signal synthesiser generating an inphase and a quadrature demodulation signal in which the receiver is arranged in operation to
 a) receive a preamble sinusoidal signal representing a symbol; 
 b) match a phase of the inphase signal generated by the signal synthesiser to a phase of the received signal; 
 c) match a phase of the quadrature signal generated by the signal synthesiser to be ninety degrees out of phase the phase with the received signal; 
 d) repeat a step of determining whether the received signal represents a symbol having positive inphase and quadrature components; in the event the inphase and quadrature components of the received signal are not both positive, adjusting the phase of the inphase and quadrature components by a fixed phase shift until the received signal represents a symbol having positive inphase and quadrature components; 
 e) determine a cross product of the received inphase and quadrature components and ideal inphase and quadrature components 
 f) index a look up table with said cross product to determine a phase adjustment and 
 g) adjust the phase of the demodulation signals in accordance with said determined phase adjustment and in which the receiver is further arranged in operation to adjust for small phase rotations due to frequency fluctuations by 
 h) determining whether the received signal represents a symbol having equal inphase and quadrature components; 
 i) in the event the received signal represents a symbol having equal inphase and quadrature components, updating a phase difference measure in dependence upon the difference between the magnitude of the inphase and quadrature component of the received signal; and 
 j) adjusting the phase of the signals generated by the signal synthesiser when said phase difference measure exceeds a predetermined threshold. 
 
     
     
       10. The apparatus according to  claim 9 , in which the receiver is arranged in operation to repeat steps e) to g) until the phase adjustment is less than a predetermined minimum. 
     
     
       11. The apparatus according to  claim 10 , in which said predetermined minimum is 0.5 degrees. 
     
     
       12. The apparatus according to  claim 9 , in which the preamble signal is a sinusoidal signal representing a symbol in the encoding scheme having the greatest signal to noise ratio. 
     
     
       13. The apparatus according to  claim 9 , in which the lookup table comprises entries for unequally spaced phase differences with entries close together for small phase differences and entries further apart for greater phase differences. 
     
     
       14. The apparatus according to  claim 9 , in which the phase difference measure is updated at step i) by incrementing the phase difference measure by one if a first one of the components is greater than a second one of the components and by decrementing the phase difference measure by one if the second one of the components is greater that the first one of the components. 
     
     
       15. The apparatus according to  claim 14  in which the predetermined threshold is fifteen. 
     
     
       16. The apparatus according to  claim 9 , in which the phase is adjusted by step j) by 0.5 degrees when said phase difference measure exceeds a predetermined threshold.